Changing the footrest to a full-width plate, angled at 60 degrees, is a game changer.
If you don’t want to go the custom bulkhead route, there are some commercial products available, I put a “big foot” foot plate in my Rapier 20. Expensive and heavier than I prefer, but robust: http://expeditionkayaks.blogspot.com/2015/03/rob-mercer-ek-bigfoot-footplate-system.html
They make a similar plate without the rudder pedals (skeg foot).
I decided not to install a full-height (hull to deck) footplate, as the half-footplate allows me to push my heels under the plate for a good stretch, which is a godsend for long hours in the cockpit. If you install a foam footrest, you might want to leave a hole in the center for the same purpose.
Most skis and “go-fast” kayaks use a 60 degree angle for the footrest. 90 is not ergonomic and is uncomfortable and lower angles (like 45 degrees) have the irritating tendency to cause your feet to slide up the ramp during leg drive.
ONNO used to sell a carbon plate that connected to small plastic footpegs (but wasn’t angled). It would be pretty easy to make something similar from plywood or glass.
Warning – any footbrace modification that you make, test it to ensure it doesn’t pose an entrapment hazard (shoes lace entanglement, feet “stuck” should you ender and slam the bow into the bottom, etc).
For seat comfort on long multi-day races, I use a pad cut from a Thermarest Z-lite foam mattress, and top with a thin teflon pad. This gives comfort for bony butts plus allows you to rotate.